Tissue engineering is the application of principles and methods of engineering and life sciences toward a fundamental understanding and development of biological substitutes to restore, maintain and improve human tissue functions.
Orthopedic management of lesions to articular cartilage remains a persistent problem for the orthopedist and patient because articular cartilage has a limited intrinsic ability to heal. This has prompted the development of numerous procedures to treat these lesions and to halt or slow the progression to diffuse arthritic changes.
Tissue engineering may eliminate many of the problems associated with current surgical options. Current tissue engineering methods are aimed at filling the cartilage defects with cells with or without scaffolds to promote cartilage regeneration. Implantation of scaffolds alone leads to a poor quality reparative tissue. Chondrocytes implanted either alone or in combination with a scaffold have failed to restore a normal articular surface, and the hyaline cartilage formed early on in response to chondrocyte-containing scaffolds seems to deteriorate with time.
Damage to the spinal cord may result in autonomic dysfunction, a loss of sensation, or a loss of mobility. Such spinal cord injury (SCI) frequently is caused by trauma, tumors; ischemia, developmental disorders, neurodegenerative diseases, demyelinative diseases, transverse myelitis, vascular malformations, or other causes. The consequences of SCI depend on the specific nature of the injury and its location along the spinal cord. In addition because SCI is a dynamic process, the full extent of injury may not be apparent initially in all acute cord syndromes. Incomplete cord lesions may evolve into more complete lesions; more commonly, the injury level rises one or two spinal levels during the hours to days after the initial event. A complex cascade of pathophysiologic events accounts for this clinical deterioration.
The psychological and social impact of SCIs often is devastating. Some of the general disabling conditions associated with SCI are permanent paralysis of the limbs, chronic pain, muscular atrophy, loss of voluntary control over bladder and bowel, sexual dysfunction, and infertility.
Recent advances in neuroscience have drawn considerable attention to research into SCI and have made significantly better treatment and rehabilitation options available. Functional electrical stimulation (FES), for example, has shown the potential to enhance nerve regeneration and allow significant improvements in restoring and improving functional capacity after SCI. However, not all patients with spinal cord injury qualify for FES (a complete lesion of the spinal cord must be established); the patient must be in a neurologically stable condition; and the peripheral nerves must be intact to respond to exogenous electrical stimulations. Therefore, tissue engineering methods that could successfully restore, maintain, and improve the damage caused by spinal cord injury would eliminate many of the problems associated with current treatment options.
The development of improved tissue regeneration strategies will require a multi-disciplinary approach combining several technologies. Due to the size and complexity of tissues such as the spinal cord and articular cartilage, specialized constructs incorporating cells as well as smart materials may be a promising strategy for achieving functional recovery.
The use of stem cells for tissue engineering therapies is at the forefront of scientific investigation. Stem cells have the ability to differentiate into various cells types and thus promote the regeneration or repair of a diseased or damaged tissue of interest.
For example, mesenchymal stem cells (MSC) are multipotent cells that are capable of differentiating along several lineage pathways. In the body, adult stem cells often are localized to specific chemically and topologically complex microenvironments, or so-called “niches”. Increasing experimental evidence supports the notion that stem cells can adjust their properties according to their surroundings and select specific lineages according to cues they receive from their niche (Xie L, Spradling, A C, “A Niche Maintaining Germ Line Stem Cells In Drosophila Ovary,” Science 290:328 (2000); Fuchs E, Segre J, “Stem Cells: A New Lease On Life,” Cell 100: 143-155 (2000), Watt F M, Hogan B L M, “Out Of Eden: Stem Cells And Their Niches,” Science 287:1427 (2000)). It follows that in order for an MSC therapy to be successful in the repair of a specific tissue type, the microenvironment of the cells should be designed to relay the appropriate chemical and physical signals to them.
A viable approach may be to mimic characteristics of the microenvironment during cartilage and neurite development may be a viable approach. During cartilage development, for example, one of the earliest events is pre-cartilage mesenchymal cell aggregation and condensation resulting from cell-cell interaction, which is mediated by cell-cell and cell-matrix adhesion (fibronectin, proteoglycans, and collagens). (DeLise A. M., Fischer L, Tuan R S, “Cellular Interactions And Signaling In Cartilage Development. Osteoarthritis and Cartilage 8: 309-334 (2000)). Type I collagen, the predominant matrix protein present in the early stages of development, is later transformed to Type II collagen by increased cell synthesis during differentiation. (Safronova E E, Borisova N V, Mezentseva S V, Krasnopol'skaya K D, “Characteristics Of The Macromolecular Components Of The Extracellular Matrix In Human Hyaline Cartilage At Different Stages Of Ontogenesis.” Biomedical Science 2: 162-168 (1991)). Multiple growth factors and morphogens are also present contributing to the regulation of the differentiation process.
A few studies have demonstrated the use of MSCs for cartilage repair through intra-articular injection and have shown promise. (Murphy M, Fink D J, Hunziker E B, Barry F P, “Stem Cell Therapy In A Caprine Model Of Osteoarthritis,” Arthritis Rheumatism 48: 3464-3474 (2003); Ponticello M S, Schinagel R M, Kadiyala, S, Barry F P, “Gelatin-Based Resorbable Spone As A Carrier Matrix For Human Mesenchymal Stem Cells In Cartilage Regeneration Therapy,” J Biomed Materials Res 52: 246-255 (2000)). The MSCs are injected at a high cell density either alone (in saline) or in combination with a gelatinous/hydrogel matrix in order to promote cell-cell aggregation. However, the use of MSCs in combination with biomaterials of varying architectures that may closely mimic the physical architecture of the native extracellular matrix during development to direct chondrogenic differentiation has yet to be investigated.
Schwann cell-laden grafts and nerve conduits have shown promise for repairing nervous tissue (Brook G A, Lawrence J M, Raisman G. Columns of Schwann cells extruded into the CNS induce in-growth of astrocytes to form organized new glial pathways. Glia 2001; 33:118-130) and optic nerves (Negishi H. Optic nerve regeneration within artificial Schwann cell graft in the adult rat. Brain Research Bulletin 2001; 55:409-419), as have injections of adult stem cells (Desawa M. Specific induction of neuronal cells from bone marrow stromal cells and application for autologous transplantation. Journal of Clinical Investigation 2004; 113:1701-1710), but the size and complexity of the spinal cord warrants the development of specialized constructs.
The present invention addresses these problems. From a biological viewpoint, almost all human tissues and organs are characterized by well-organized hierarchical fibrous structures through the assembly of nanoscale elements. It is believed that converting biopolymers into fibers and networks that mimic native structures will ultimately enhance the utility of these materials as scaffolds. Nanoscale fibrous scaffolds may provide an optimal template for stem cell growth, differentiation, and host integration.
It is known that cells will attach to synthetic polymer scaffolds leading to the formation of tissue. (Sachlos, E. and Czernuszka, Eur. Cells & Materials 5: 29-40 (2003)). Using fetal bovine chondrocytes maintained in vitro, Li et al. have shown that scaffolds constructed from electrospun three-dimensional nanofibrous poly(ε-capro-lactone) act as a biologically preferred scaffold/substrate for proliferation and maintenance of the chondrocyte phenotype. (Wan-Ju Li, et al., J. Biomed. Mater. Res. 67A: 1105-1114 (2003)).
Because electric polarization can influence cell growth and behavior, e.g., growth of different cell types (Yang, X. L. et al., J. Mater. Sci.: Mater. Med. 17: 767 (2006)), enhancement of nerve regeneration (Kotwal, A. et al., Biomaterials, 22: 1055 (2001)), and cell adhesion and morphology (Kapurr., et al., J. Biomed Mater. Res. 27: 133 (1996)), it was hypothesized that a three-dimensional, electrically charged polymer scaffold would be a promising approach for a number of tissue engineering applications (e.g., nerve, bone, cartilage regeneration).